The principle of these field effects transistors is well known.
Thus, reference can be made to IEEE Transactions on Electron Devices, vol. ED-29, No. 6, June 1982, D. Delagebeaudeuf and N. T. Linh, "Metal-(n) AlGaAs-GaAs two-dimensional electron gaz FET", pp. 955-960 describing the operation of a TEGFET and IEEE Transactions on Electron Devices, vol. 35, No. 7, July 1988, M. C. Foisy et al., "The role of inefficient charge modulation in limiting the current-gain cutoff frequency of the MODFET", pp. 871-878 describing the operation of a MODFET.
This type of structure is obtained by stacking semiconductor layers of different types, which can be doped or undoped and which are deposited on a generally semiinsulating substrate material. Operation is based on the principle of transferring electric charges (electrons or holes) from certain doped layers, constituting the mobile electric charge donor layers, to other doped or undoped layers constituting the conduction channel or channels of the transistor. A metal gate placed on the surface of the semiconductor structure makes it possible to modify the charge carrier concentration in each of these layers and control the conduction properties of the component. On said semiconductor layer stack there are also two sources and drain metal contacts on either side of the gate.
In this type of structure, one of the essential factors for improving the performance characteristics, particularly in ultra-high frequency, is the spatial distribution of the mobile charge carriers, electrons or holes.
The volume of the component is subdivided into two subdomains on the one hand the electric conduction channels (SD1) and on the other the other semiconductor layers (SD2), such as the mobile electric charge donor layers and the buffer layers generally interpose between the substrate and the conduction channels.
The concentration of mobile charges in one of these subdomains of the component is defined as being, for a given miniband, the product of the concentration of the carriers for said miniband by the presence probability of a carrier of said miniband in the considered subdomain. The total concentration of mobile charges in the subdomain SD1 is called NS1 and that in the subdomain SD2 is called NS2.
1) Cutoff frequency
The cutoff frequency Fc of the current gain of the component can be considered as proportional to the ratio: ##EQU1## in which Vg is the potential applied to the gate.
No matter what the considered application (low noise transistor, power transistor, etc.) for a given operating frequency and therefore a fixed maximum ratio dNS2/dNS1, an attempt is always made to have the largest possible range of values for NS1, i.e. 0&lt;NS1&lt;N1lim is the maximum limit value of NS1. However, on modifying the gate voltage Vg for increasing NS1, there is a simultaneous increase of the ratio dNS2/dNS1.
Various means have been studied for improving the performance characteristics of field effect transistors, via an increase in the discontinuity of the bands (conduction it transport is by electrons, if not valency).
In order to improve the efficiency of the charge transfer and reduce the dNS2/dNS1 with given NS1, two solutions have been envisaged:
a--modifying the mobile charge donor layer or layers (or subdomain SD2),
b--modifying the material in which conduction takes place (or subdomain SD1).
a--The alloys of doped III-V or II-VI materials commonly sued as charge donor layers (SD2) introduce carrier traps linked with doping atoms, such as the centre DX in III-V alloys containing aluminum. The binding energy is then high, which limits the efficiency of the transfer of charges from layers SD2 to layers SD1.
It is the existence of these localized states (or trapped carriers) in the subdomain SD2, which limits the performance characteristics of the transistors produced on GaAs or InP substrate, such as those described in the document by L. D. Nguyen et al., Proc. IEEE/Cornell Conf. 1987, pp. 60-69 "AlGaAs/InGaAs modulation-doped field effect transistors (MODFET's)".
On a GaAs substrate, these effects would seem to be limited for the alloy In.sub.0.51 Ga.sub.0.49 P adapted from the crystal lattice standpoint, but the height of the heterojunction is less than that accessible in the AlGaAs/GaAs system.
In the case of AlGaAs/GaAs structures, it has also been considered to substitute the ternary alloy AlGaAs by a relatively short period superlattice, typically below 5 nm. This superlattice consists of an alternation of thin GaAs and AlAs films. Only the GaAs films are doped. Such a structure is described in Proc. IEEE/Cornell Conf., pp. 199-208, 1985, by L. H. Camnitz "The role of charge control on drift mobility in AlGaAs/GaAs MODFET's".
The object of the sublattice is to obtain a material, whose band structure is very close to that of the ternary alloy AlGaAs, but where the bonding energy of the charge donor impurities is low. This procedure should, a priori, make it possible to increase the mean aluminum composition of the charge donor layers by eliminating the centre DX and therefore increasing the efficiency of the transfer of the mobile charges from the charge donor layers to the conduction layers. However, this solution has not led to any improvement and has consequently not been adopted by field effect transistor designers and manufacturers.
b--With regards to the modification of the material used for conduction, subdomain SD1, the alloys of In.sub.x Ga.sub.1-x As, mismatched with respect tot he lattice parameter, have been widely used both on a GaAs substrate with 0&lt;x.ltoreq.1 and on an InP substrate with 0.35&lt;x.ltoreq.1, respectively in place of GaAs and In.sub.0.53 Ga.sub.0.47 As.
Reference can be made in this connection to the article by N. Moll et al., IEEE Transactions on Electron Devices, vol. 35, No. 7, July 1988, pp. 879-886, "Pulse-doped AlGaAs/InGaAs pseudomorphic MODFET's", relative to the use of In.sub.0.25 GA.sub.0.75 As on GaAs.
InGaAs alloys have a smaller forbidden energy band than that of GaAs or In.sub.0.53 Ga.sub.0.47 As, so that it is possible to increase the band discontinuities. Their crystal mesh parameters are, however, different from those of GaAs and InP and consequently there are constrains in connection therewith. Therefore this limits the usable indium composition range, typically 0&lt;x&lt;0.25 on GaAs and 0.53&lt;x&lt;0.65 on InP.
Thus, the limitation of the number of materials, whose lattice parameter is sufficiently close to that of the nominal or base material makes it impossible to find novel combinations reducing the ratio dNS2/dNS1.
2) High current applications
The drain current available in such transistors is typically the product of NS1lim by the average speed of the carriers beneath the gate. For a given form, the latter is essentially dependent on the nature of semiconductor layers. The possible solutions and their limits are given in paragraphs a nd b.
3.) Linearity
In these transistors, an attempt is also made to obtain an as linear as possible current-voltage response. However, the linearity of the response of a transistor is affected by the modification of the distribution of the electrical charges in the structure between the subdomains (SD1 and SD2) and within the subdomain SD1.
More precisely, the presence probability densities associated with each of the minibands are deformed with the change of voltage applied to the gate, which leads to a modification in the average position of the electrons relative to the gate. This introduces a non-linear voltage variation of the carrier concentration in the conduction channels. The use of AlGaAs/InGaAs/AlGaAs structures in place of AlGaAs/InGaAs/GaAs structures has already made it possible to limit this effect. In this connection reference can be made to the article by C. Gaonach et al., Proc. GaAs and related Compounds, Jersey, 1990, "Characterization of pseudomorphic HEMT structures AlGaAs/InGaAs/AlGaAs".
With a view to optimizing the dNS2/dNS1 ratio in the HEMT, use has also been made of a local disturbance of the potential energy of the structure in order to selectively modify the self-energies and wave functions at the considered point of the studies minibands, liable to be populated by carriers. Thus, the theory of disturbances in quantum mechanics shows that the effect of introducing such as localized potential disturbance causes a displacement of the self-energy which increases with the overlap of said potential with the presence probability in the undisturbed system. Such a localized disturbance is obtained by changing the material of a thin film of the structure.
Thus, it has been envisaged to introduce an InAs monolayer into the conduction channel of the AlGaAs/GaAs and AlGasAs/InGaAs structures producing a potential well. This is described in Japanese Journal of Applied Physics, vol. 30, No. 2A, February 1991, pp. L166-L169, "a new high electron mobility transistor (HEMT) structure with a narrow quantum well formed by inserting a few monolayers in the channel" by K. Matsumura et al.
The introduction of this potential well at a given point increases the presence probability at this point and increases its population by reducing its energy, particularly if the presence probability density is high at this point prior to the disturbance.
Moreover, FR-A-2,646,290 envisages the introduction of a thin AlInAs film, doped on the border of the conduction channel of an AlGaAs/InGaAs/GaAs structure in order to create a tunnel barrier between the conduction stack and the donor stack, to avoid the passages of electrons from the former to the latter (dynamic effect). However, in actual fact this concept failed.